[Show abstract][Hide abstract] ABSTRACT: Electron density is a fundamental quantity that enables understanding of the chemical bonding in a molecule or in a solid and the chemical/physical property of a material. Because electrons have a charge and a spin, two kinds of electron densities are available. Moreover, because electron distribution can be described in momentum or in position space, charge and spin density have two definitions and they can be observed through Bragg (for the position space) or Compton (for the momentum space) diffraction experiments, using X-rays (charge density) or polarized neutrons (spin density). In recent years, we have witnessed many advances in this field, stimulated by the increased power of experimental techniques. However, an accurate modelling is still necessary to determine the desired functions from the acquired data. The improved accuracy of measurements and the possibility to combine information from different experimental techniques require even more flexibility of the models. In this short review, we analyse some of the most important topics that have emerged in the recent literature, especially the most thought-provoking at the recent IUCr general meeting in Montreal.
[Show abstract][Hide abstract] ABSTRACT: Professor Dunitz questions the usefulness of ascribing crystalline structural stability to individual atom-atom intermolecular interactions viewed as bonding (hence stabilizing) whenever linked by a bond path. An alternative view is expressed in the present essay that articulates the validity and usefulness of the bond path concept in a crystallographic and crystal engineering context.
[Show abstract][Hide abstract] ABSTRACT: High-resolution crystal structure determination and spherical and multipolar refinement enabled an organic solid solution of 1-(4′-chlorophenyl)-2-methyl-4-nitro-1
-imidazole-5-carbonitrile and 5-bromo-1-(4′-chlorophenyl)-2-methyl-4-nitro-1
-imidazole to be found, which would not normally be revealed using only standard resolution data (
0.8 Å), as the disordered part is only visible at high resolution. Therefore, this new structure would have been reported as just another polymorphic form, even more reasonably as isostructural with other derivatives. To the best of our knowledge this is the first example of organic solid solution modelled
charge density Hansen–Coppens formalism and analysed by means of quantum theory of atoms in molecules (QTAIM) theory.